1. Field of the Invention
The present invention relates to digital watermarking of moving-picture data and in particular to a method and system for inserting watermark data into the moving-picture data.
2. Description of the Prior Art
With wide-spreading digital satellite broadcasting, Internet transmission and DVD (Digital Video Disk), etc. in recent years, digital images are becoming easily accessible to users. Since the quality of digital images does not deteriorate when they are copied, securing their copyrights has been an important issue.
For purposes of securing copyright protection for MPEG (moving picture experts group) data, there have been disclosed a watermarking system for embedding a plurality of watermarks into DCT (discrete cosine transform) coefficient domain of MPEG data and a watermark detecting system for extracting and detecting the embedded watermarks from the watermarked MPEG data (see Japanese Patent Application Unexamined Publication No. 11-55639).
More specifically, a plurality of watermarks different from each other are previously stored each corresponding to picture types (I-, B-, and P-pictures in MPEG). In the watermarking system, one watermark is selected from the plurality of watermarks depending on the type of a picture to be encoded and is embedded into the image data in units of 8xc3x978 DCT coefficient block.
Assuming that f(1), f(2), . . . , f(n) are DCT coefficients listed in frequency-ascending order, a watermarked coefficient F(i) is calculated by the following equation:
F(i)=f(i)+xcex1xc3x97avg(f(i))xc3x97w(i),
where i=1, 2, 3, . . . , n, w(i) is an element of a water mark selected according to a normal distribution with mean mx=0 and variance "sgr"2=1, xcex1 is a scaling element, and avg(f(i)) represents a local average over three DCT coefficients in the neighborhood, which is calculated by the following form:
avg(f(i))=(|f(ixe2x88x921)|+|f(i)|+|f(i+1)|)/3.
On the other hand, the watermark detecting system can detect an embedded watermark without the need of the original image data. The watermarked MPEG data is decoded and a watermark is extracted from the DCT components of the decoded data using the local average over the neighborhood. A watermark element W(i) is extracted by calculating the following form: F(i)/avg(F(i)). A watermark element W(i) is accumulated in one frame/field to produce WF(i).
A statistical similarity C between w(i) and WF(i) can be calculated using vector inner product as follows:
C=WFxc3x97w/(|WF|xc3x97|w|)
where WF=(WF(1), WF(2), . . . , WF(n)) and w=(w(1), w(2), . . . , w(n)).
If the statistical similarity C calculated as described above exceeds a predetermined threshold, it is determined that the corresponding watermark is embedded in the MPEG data.
However, the inventor found that a degree of embedded watermark effect on MPEG data is varied depending on the type of a picture in MPEG. If watermark frequency coefficients having a certain amplitude (level) is uniformly embedded to MPEG data regardless of the type of a picture as the prior art, then there are cases where the quality of image is deteriorated and the embedded watermark is hard to be detected.
More specifically, when the watermark is strongly embedded to the MPEG data, ease of watermark detection is enhanced but the quality of image is deteriorated. Contrarily, when the watermark is lightly embedded to the MPEG data, the quality of image is kept sufficiently but the embedded watermark becomes hard to be detected. In other words, the quality of a watermarked image is traded off against watermark detection efficiency.
It is an object of the present invention to provide a watermarking system and method allowing both sufficiently high quality of watermarked moving pictures and enhanced ease of watermark detection.
According to the present invention, a method for inserting a watermark into a selected block of frequency component data in a picture of data having one of a plurality of picture types, includes the steps of: determining a picture type of the selected block of frequency component data; determining a watermark level depending on the picture type of the selected block of frequency component data; and inserting a watermark having the watermark level into the selected block of frequency component data.
The picture types are preferably Intra-frame coding picture (hereafter, I-picture), Inter-frame Predictive coding picture (hereafter, P-picture) and Bidirectionally Predictive coding picture (hereafter, B-picture), wherein a relative proportion of watermark levels of I-picture, B-picture, and P-picture is 10:7:5.
As described above, a watermark level of a watermark to be embedded into a picture of data is changed depending on the picture type of a selected block of frequency component data, and then a watermark having the watermark level is inserted into the selected block of frequency component data. Therefore, a moving-picture can be effectively embedded with a digital watermark without substantially reducing the quality of image and the digital watermark can be detected reliably.
According to an embodiment of the present invention, a method includes the steps of: storing a plurality of watermarks for each picture type, each of the watermarks corresponding to a different block of the picture of data; determining a picture type of the selected block of frequency component data; determining a watermark level depending on the picture type of the selected block of frequency component data; selecting a watermark from the plurality of watermarks depending on which block is selected; and inserting a selected watermark having the watermark level into the selected block of frequency component data.
According to another embodiment of the present invention, a method includes the steps of: a) storing a plurality of watermarks for each of the picture types, wherein a watermark level of the watermarks varies depending on a picture type; b) selecting a watermark of a type depending on a picture type of the selected block of frequency component data; and c) inserting a watermark of a selected type into the selected block of frequency component data.
According to still another embodiment of the present invention, a method includes the steps of: a) storing a reference watermark; b) determining a picture type of the selected block of frequency component data; c) changing a watermark level of the reference watermark depending on the picture type of the selected block of frequency component data to produce a watermark to be used; and d) inserting the watermark to be used into the selected block of frequency component data.
In the step (c), the watermark level of the reference watermark may be changed depending on both the picture type of the selected block of frequency component data and the selected block.
According to another aspect of the present invention, a method for detecting a watermark from a selected block of frequency component data in a picture of data having one of a plurality of picture types, includes the steps of: storing a plurality of watermarks for each of the picture types, wherein a watermark level of the watermarks varies depending on a picture type; extracting a watermark from the selected block in the picture of data; selecting a watermark of a type depending on a picture type of the selected block; and determining whether the stored watermark is embedded in the selected block, based on an extracted watermark with a selected watermark.
According to the present invention, a system for inserting a watermark into a selected block of frequency component data in a picture of data having one of a plurality of picture types, includes: a table storing a plurality of watermarks for each of the picture types, wherein a watermark level of the watermarks varies depending on a picture type; a selector for selecting a watermark of a type depending on a picture type of the selected block of frequency component data; and an inserting section for inserting a selected watermark of a selected type into the selected block of frequency component data.
According to the present invention, a system includes: a memory storing a reference watermark; a determiner for determining a picture type of the selected block of frequency component data; a multiplier for multiplying a watermark level of the reference watermark by a factor varying depending on the picture type of the selected block of frequency component data to produce a watermark to be used; and an inserting section for inserting the watermark to be used into the selected block of frequency component data.
The multiplier may multiply a watermark level of the reference watermark by a factor varying depending on both the picture type of the selected block of frequency component data and the selected block.
The picture types are preferably Intra-frame coding picture (hereafter, I-picture), Inter-frame Predictive coding picture (hereafter, P-picture) and Bidirectionally Predictive coding picture (hereafter, B-picture), wherein a relative proportion of factors corresponding to I-picture, B-picture, and P-picture is 10:7:5.